Calculate monthly or annual pond evaporation loss from surface area, pan evaporation rate, and pan coefficient. Plan water budgets accurately.
Open water surfaces lose significant volumes of water to evaporation, especially in hot, dry, and windy climates. For farm ponds, stock tanks, and irrigation reservoirs, evaporation can represent 3–7 feet of water loss per year in arid regions.
Evaporation is typically estimated from pan evaporation data (available from weather stations) multiplied by a pan coefficient (usually 0.70–0.80) to convert from the small pan to a larger open water body. Multiplying the adjusted evaporation depth by surface area gives the volume of water lost.
This calculator estimates evaporation loss in depth, volume, and percentage of pond storage so you can include it in your water budget and plan accordingly. Whether you are a beginner or experienced professional, this free online tool provides instant, reliable results without manual computation. By automating the calculation, you save time and reduce the risk of costly errors in your planning and decision-making process. This tool handles all the complex arithmetic so you can focus on interpreting results and making informed decisions based on accurate data.
Ignoring evaporation can cause unexpected water shortages, especially for small ponds in hot climates. This tool quantifies the loss so you can plan supplemental supply or reduce surface area. Having a precise figure at your fingertips empowers better planning and more confident decisions. Manual calculations are error-prone and time-consuming; this tool delivers verified results in seconds so you can focus on strategy.
Evaporation Depth = Pan Evaporation × Pan Coefficient Volume Lost (ac-ft) = Surface Area (ac) × Evaporation Depth (ft) Volume Lost (gal) = ac-ft × 325,851
Result: Annual Loss = 7.0 ac-ft (2,281,000 gal)
Evaporation depth = 60 in × 0.70 = 42 in = 3.5 ft. Volume lost = 2.0 ac × 3.5 ft = 7.0 ac-ft = 2,280,957 gallons per year.
The highest evaporation rates in the U.S. occur in the desert Southwest (70–90 in/yr), followed by the southern Great Plains (55–70 in/yr). The upper Midwest and Northeast have the lowest rates (25–40 in/yr). NOAA publishes maps of mean annual lake evaporation by region.
Floating covers (HDPE, foam, or shade cloth) can reduce evaporation by 70–90%. Windbreaks (trees or fabric) reduce wind speed and can cut evaporation by 10–25%. Chemical monolayers (hexadecanol) can reduce evaporation by 20–40% but require regular reapplication.
A complete water budget: Inflow (runoff + groundwater + direct precipitation) – Outflow (evaporation + seepage + withdrawals) = Change in Storage. Balance this equation monthly to predict when the pond will be full, adequate, or critically low.
A Class A evaporation pan overestimates open-water evaporation because it's small, exposed on all sides, and heats up quickly. The pan coefficient (0.70–0.80) adjusts for this difference.
In the central U.S., annual lake evaporation is about 40–55 inches. In the arid Southwest, it can exceed 70 inches. A 2-acre pond losing 50 in/yr loses about 8.3 ac-ft.
Yes. Windbreaks, shade structures, floating covers, and shade balls all reduce evaporation. Minimizing surface area relative to volume (deeper ponds) also helps.
Higher humidity reduces the vapor pressure deficit, slowing evaporation. Humid regions may lose 30–40 in/yr vs 60–80 in/yr in arid areas.
Yes. Net evaporation = gross evaporation – direct precipitation. In humid regions, net evaporation may be only 10–20 in/yr because rainfall partially offsets losses.
Seepage is separate from evaporation. Clay-lined ponds may lose 0.1–0.5 in/day to seepage; sandy soils can lose 1–3 in/day without a liner. Both should be included in the water budget.